The present invention relates to RF (radio frequency) communication systems, and more specifically, to apparatus and methods for reducing transmit band noise floor and ACP (Adjacent Channel Power).
Wireless communication, such as cell phones for voice and data, has become extremely popular. Currently, several wireless schemes are in use, including GSM (Group Special Mobile), TDMA (Time Division Multiple Access), and CDMA (Code Division Multiple Access). Of these, CDMA appears to be emerging as the standard in the U.S., European and Asian markets. CDMA often requires RF transmissions using both phase and amplitude modulation. The efficiency and power consumption of the power linear amplifiers used to generate an RF signal in either a CDMA cell phone or base station are therefore extremely important.
Use of low efficiency linear amplifiers is detrimental for several reasons. Such amplifiers tend to burn a significant amount of energy which is problematic, particularly in a battery operated cell phone. Power consumption is also problematic in base stations. The heat caused by many low efficiency amplifiers in a base station can cause components to fail, thus reducing reliability. The linearity of the power amplifier is also important. In a base stations where the transmission of multiple signals occurs simultaneously, amplifiers characterized by poor linearity may cause the inadvertent mixing of these signals.
A number of types of amplifier classes can be used in RF communication systems, including Class A, Class AB, Class C, Class E, Class F, and Class D (sometimes referred to as digital amplifiers). Each of these types of amplifiers, however, have significant problems when operating in the RF range. For example, Class A and Class AB amplifiers have very poor efficiency but reasonable linearity. Class C amplifiers are reasonably efficient but are only practical for phase modulation. Similarly Class E, F, and D amplifiers are typically only useful for phase modulation applications. Class E amplifiers have improved power efficiency when compared to C type amplifiers, but large voltage swings at their output limit their usefulness. Class F amplifiers exhibit relatively efficient switching characteristics with a repeating input signal. But with a non-repeating input signal, such as those normally encountered in a cellular phone or base station, the problems caused by harmonics become overwhelming.
Conventional class D amplifiers have linear operating characteristics and are generally highly efficient at lower frequencies but have heretofore been subject to several drawbacks at higher frequencies. Most notably, at higher frequencies such as RF they exhibit switching problems at their output transistors. As these transistors switch on and off rapidly, switching transients including high levels of current and voltage are developed at the output, causing overshoot and undershoot.
Another problem with conventional class D amplifiers when used in communication systems where RF signals are both transmitted and received is the xe2x80x9cleakagexe2x80x9d of energy from the transmit band into the receive band. This may occur if the duplexor or T/R switch at the antenna does not completely isolate the signals received at the communication device from the transmit circuitry within the device.
Most cellular systems today use frequency division duplexing (FDD) to achieve simultaneous transmit and receive capability. This is accomplished by using separate frequency bands for transmitting and receiving. For example, IS-95 CDMA systems in the United States uses 824-849 MHz for transmitting from a mobile station (i.e., upstream transmission) and 869-894 MHz for receiving at the mobile station (i.e., downstream transmission). FDD systems require limits on transmit emissions in the receive band to avoid corresponding degradation of the sensitivity of their own and neighboring mobile receivers. Systems which employ time division duplexing (TDD) also require limits on transmit emissions in the receive band, but typically to a lesser extent.
Power control is essential for most current transmitters for two reasons. First, transmitting only the minimum amount of power required to reach the desired receiver with a fixed sensitivity saves battery life by reducing the average power transmitted. Second, in systems using CDMA (Code Division Multiple Access), the capacity of a given system can be maximized by minimizing interference to other users by accurate power control. In conventional (Class A and AB) power amplifiers, the ACP (Adjacent Channel Power) decreases as a function of the output power until it hits the noise floor of the amplifier, which is a function of the DC bias current. In class-T amplifiers, the ACP at maximum is a function of the output power. At slightly lower output power levels, the quantization noise floor limits the ACP and transmit noise floor to a value determined by loop filter coefficients and sampling frequency fs. This is undesirable at significantly backed off power levels (for example, 10 dB below maximum) since ACP needs to track transmitted power in-band to maximize capacity.
In view of the foregoing, amplifiers and methods capable of reducing transmit band noise floor and ACP with power backoff are needed.
This invention addresses the needs indicated above by providing an amplifying device which includes first and second first-order filters, first and second multipliers, a quantizer, and a driver. In one embodiment, the first and second first-order filters are coupled in series. The first multiplier multiplies a signal from the first first-order filter by a coefficient k1, and the second multiplier multiplies a signal from the second first-order filter by a coefficient k2. The quantizer quantizes a summation of the signals from the first and second multipliers into one of two values, thereby generating a quantized signal. The driver amplifies the quantized signal and generates an output signal. At least one of the coefficients k1 and k2 is dynamically adjusted based on a level of the output signal.
In a further specific embodiment, a ratio of the coefficient k2 to the coefficient k1 for a first range of the level of the output signal is smaller than a ratio of the coefficient k2 to the coefficient k1 for a second range of the level of the output signal, where the first range of the level of the output signal is higher than the second range of the level of the output signal.
One aspect of the present invention provides an amplifying device which further includes a third first-order filter and a third multiplier. The first, second and third first-order filters are coupled in series. The first multiplier multiplies a signal from the first first-order filter by a coefficient k1, the second multiplier multiplies a signal from the second first-order filter by a coefficient k2, and the third multiplier multiplies a signal from the third first-order filter by a coefficient k3. The quantizer quantizes a summation of the signals from the first, second and third multipliers into one of two values. At least one of the coefficients k1, k2 and k3 is dynamically adjusted based on a level of the output signal.
In a further specific embodiment, a ratio of the coefficient k3 to the coefficient k1 for a second range of the level of the output signal is smaller than a ratio of the coefficient k3 to the coefficient k1 for a third range of the level of the output signal, where the second range of the level of the output signal is higher than the third range of the level of the output signal. A ratio of the coefficient k2 to the coefficient k1 for a first range of the level of the output signal is smaller than a ratio of the coefficient k2 to the coefficient k1 for a second range of the level of the output signal, where the first range of the level of the output signal is higher than the second range of the level of the output signal.
According to an alternative embodiment of the present invention, the amplifying device includes first-order, second-order and third-order filters instead of the three first-order filters. The first-order, second-order and third-order filters are coupled in parallel. The first multiplier multiplies a signal from the first first-order filter by a coefficient k1, the second multiplier multiplies a signal from the second first-order filter by a coefficient k2, and the third multiplier multiplies a signal from the third first-order filter by a coefficient k3. At least one of the coefficients k1, k2 and k3 is dynamically adjusted based on a level of the output signal.
According to another specific embodiment of the present invention, the quantizer has characteristics in which the quantized signal does not fluctuate between the two values when the summation is substantially stable. In other words, the quantizer has hysteresis or memory characteristics.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.